For the past 25 years, The Oprah Show has made saying two little words—"thank you"—into an art form. "We believe in the power of gratitude," Oprah says. In honor of Thanksgiving Day, Oprah is looking back at some of her favorite thank-yous!

Shortly after the 9/11 terror attacks, Miller, a firefighter from Tennessee, was sent to help search for victims' remains at the Pentagon. Miller says the devastation was depressing him...until he read a letter from a 12-year-old girl named Alex, thanking him for everything he'd done. Miller then returned the favor, thanking Alex for her inspiring letter.

Alex, now a senior at American University in Washington, D.C., says Miller has become a part of her family. She says she vividly remembers writing the letter that ended up in Miller's hands. "After having met Miller, I really thought about the importance of saying thank you," she says. "Because I think we oftentimes forget just saying those simple words can actually touch someone."